In udp_append() and udp_input(), make use of read locking on incpbs rather than write locking: while we need to maintain a valid reference to the inpcb and fix its state, no protocol layer state is modified during an IPv4 UDP receive -- there are only changes at the socket layer, which is separately protected by socket locking.
In udp_append() and udp_input(), make use of read locking on incpbs
rather than write locking: while we need to maintain a valid reference
to the inpcb and fix its state, no protocol layer state is modified
during an IPv4 UDP receive -- there are only changes at the socket
layer, which is separately protected by socket locking.

While parallel concurrent receive on a single UDP socket is currently
relatively unusual, introducing read locking in the transmit path,
allowing concurrent receive and transmit, will significantly improve
performance for loads such as BIND, memcached, etc.

MFC after:	2 months
Tested by:	gnn, kris, ps

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase